Effects of local density on pollination and reproduction in Delphinium nuttallianum and Aconitum columbianum (Ranunculaceae)

Plant populations vary in density both naturally and as a consequence of anthropogenic impacts. Density in turn can influence pollination by animals. For example, plants in dense populations might enjoy more frequent visitation if pollinators forage most efficiently in such populations. We explored effects of plant density on pollination and seed set in the larkspur Delphinium nuttallianum and monkshood Aconitum columbianum. At our site in the Colorado Rocky Mountains, flowers of D. nuttallianum are pollinated primarily by queen bumble bees, solitary bees, and hummingbirds, whereas those of A. columbianum are pollinated primarily by queen and worker bumble bees. We found that the quantity of pollination service to both species (pollinator visitation rate and pollen deposition) was at best weakly related to density. In contrast, seed set declined by approximately one-third in sparse populations relative to nearby dense populations. This decline may stem from the receipt of low-quality pollen, for example, inbred pollen. Alternatively, sparsity may indicate poor environmental conditions that lower seed set for reasons unrelated to pollination. Our results demonstrate the value of simultaneously exploring pollinator behavior, pollen receipt, and seed set in attempting to understand how the population context influences plant reproductive success.